Wideband Envelope Control in Polar Modulators

ABSTRACT

A wideband envelope modulator comprises a direct current (DC)-to-DC switching converter connected in series with a linear amplitude modulator (LAM). The DC-DC switching converter includes a pulse-width modulator that generates a PWM signal with modulated pulse widths representing a time varying magnitude of an input envelope signal or a pulse-density modulator that generates a PDM signal with a modulated pulse density representing the time varying magnitude of the input envelope signal, a field-effect transistor (FET) driver stage that generates a PWM or PDM drive signal, a high-power output switching stage that is driven by the PWM or PDM drive signal, and an output energy storage network including a low-pass filter (LPF) of order greater than two that filters a switching voltage produced at an output switching node of the high-power output switching stage.

BACKGROUND OF THE INVENTION

To achieve high energy efficiency many modern radio frequency (RF)transmitters employ what is known as a “polar modulator.” As illustratedin FIG. 1, the principal components of a polar modulator 100 include: aswitch-mode power amplifier (SMPA) 102, a phase modulator 104, and adynamic power supply (DPS) 106. Operating in the polar domain, the DPS106 modulates a (DC) power supply voltage VDD(DC) by an input amplitudemodulating signal AM(t), to produces a time varying DPS voltage VDD(t)that follows AM(t). Meanwhile, the phase modulator 104 modulates an RFcarrier by an input phase modulating signal PM(t), to produce aphase-modulated RF carrier. The SMPA 102 serves to translate thephase-modulated RF carrier to higher RF power, so that it can then beradiated over the air to a remote receiver. The RF output power P_(OUT)produced by an SMPA is proportional to the square of the magnitude ofits DPS voltage, i.e., P_(OUT) ∝ VDD²(t). This dependency is exploitedby the polar modulator 100 to superimpose the amplitude modulationcontained in the original input amplitude modulating signal AM(t) ontoSMPA's 102's RF output RF_(OUT). In other words, as the SMPA 102translates the phase-modulated RF carrier to higher RF power, it alsomodulates the RF output RF_(OUT) by the AM carried by the DPS voltageVDD(t) to produce a final phase-modulated RF output RF_(OUT) having a‘signal envelope’ that follows the original input amplitude modulatingsignal AM(t).

Although RF transmitters constructed from polar modulators can be madeto operate with high energy efficiencies, one problem that arises fromoperating in the polar domain is that the bandwidth of the amplitudemodulating signal AM(t) can be high, depending on the modulation schemebeing used. For example, the modulation schemes used in Wideband CodeDivision Multiple Access (W-CDMA) and Long-Term Evolution (LTE) systemsproduce amplitude modulating signals AM(t) that tend to inflect abruptlywhen traversing to and from zero magnitude. As illustrated in FIG. 2,these sharply inflecting low-magnitude events 202 have substantial highfrequency content, so when represented in the frequency domain are seento have a very wide bandwidth. In order to accurately reproduce thelow-magnitude events in the signal envelope of the RF output RF_(OUT),the DPS 106 must therefore have a wide operating bandwidth capability,preferably at least 5× to 10× the modulated signal bandwidth.Unfortunately, a conventional DPS lacks this capability. It would bedesirable, therefore, to have a polar modulator with a DPS that iscapable of operating over a wide envelope bandwidth. It would be furtherdesirable to have a DPS that has that same capability and that alsooperates with high energy efficiency.

BRIEF SUMMARY OF THE INVENTION

A wideband envelope modulator for a polar modulator is disclosed. Thewideband envelope modulator comprises a direct current (DC)-to-DCswitching converter connected in series with a linear amplitudemodulator (LAM). The DC-DC switching converter includes a pulse-widthmodulator that generates a PWM signal with modulated pulse widthsrepresenting a time varying magnitude of an input envelope signal or apulse-density modulator that generates a PDM signal with pulse widthsthe vary over time to represent the time varying magnitude of the inputenvelope signal, a field-effect transistor (FET) driver stage thatgenerates a PWM or PDM drive signal, a high-power output switching stagethat is driven by the PWM or PDM drive signal, and an output energystorage network including a low-pass filter (LPF) of order greater thantwo that filters a switching voltage produced at an output switchingnode of the high-power output switching stage. In a preferred embodimentof the invention the DC-DC switching converter is configured to operateopen loop, so that the frequency response of the DC-DC switchingconverter remains uncompromised, and the LAM comprises a gallium nitridehigh electron mobility transistor (GaN HEMT) pass transistor controlledby a silicon-opamp error amplifier, which together allow the LAM torealize an operating bandwidth in excess of 900 MHz.

Further details of the invention, including a detailed description ofthe above-summarized and other exemplary embodiments of the invention,will now be described with reference to the accompanying drawings, inwhich like reference numbers are used to indicate identical orfunctionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified drawing of a conventional polar modulator,highlighting its principal components;

FIG. 2 is a signal diagram of an amplitude modulating signal AM(t),illustrating how abrupt changes in its waveform result in high frequencycontent;

FIG. 3 is drawing depicting a polar modulator with a wideband envelopemodulator, according to one embodiment of the present invention;

FIG. 4 is drawing showing in more detail the DC-DC switching converterof the wideband envelope modulator of the polar modulator depicted inFIG. 3, in accordance with one embodiment of the invention;

FIG. 5 is a drawing illustrating one way the FET driver stage of theDC-DC switching converter depicted in FIG. 4 is implemented in oneembodiment of the invention;

FIG. 6 is a signal diagram illustrating how the diodes in each of thedrive interfaces in the FET drive stage depicted in FIG. 5 operate toclamp the input PWM waveform between a fixed input-high drive levelV_(gs,H) and a fixed input-low level V_(gs,L);

FIG. 7 is a frequency response diagram illustrating how the fourth-orderlow-pass filter (LPF) in the output energy storage network of the DC-DCswitching converter depicted in FIG. 4 provides the DC-DC switchingconverter the ability to achieve a significantly wider output dynamicfrequency response compared to if it employed only a second-order filterwith the same switching noise suppression; and

FIG. 8 is a drawing showing one way that the linear amplitude modulator(LAM) in the wideband envelope modulator of the polar modulator depictedin FIG. 3 is constructed in one embodiment of the invention.

DETAILED DESCRIPTION

Referring to FIG. 3, there is shown a polar modulator 300 according toone embodiment of the present invention. The polar modulator 300comprises a switch-mode power amplifier (SMPA) 302, a phase modulator304, and a dynamic power supply (DPS) 306 that includes a direct-current(DC)-to-DC switching converter 308 connected in series with a linearamplitude modulator (LAM) 310. In the description below, the DPS 306 isalso referred to as the “series envelope modulator” 306. Because of itsability to operate over a wide frequency range, it may also be referredto as the “wideband envelope modulator” 306. Similar to as in theconventional polar modulator 100, the phase modulator 304 in the polarmodulator 300 serves to modulate an RF carrier by an input phasemodulating signal PM(t), to produce a phase-modulated RF carrier, andthe SMPA 102 serves to translate the phase-modulated RF carrier tohigher RF power. However, unlike the conventional polar modulator 100,the polar modulator 300 includes the series envelope modulator 306,which, for reasons that will become more clear from the detaileddescription that follows, allow it to achieve a wide output dynamicrange and operate with high energy efficiency over a wide frequencyrange.

FIG. 4 is a drawing illustrating how the DC-DC switching converter 308is constructed in one embodiment of the invention. The DC-DC switchingconverter 308 comprises a pulse-width modulator (PWM) 402 (or,alternatively, a pulse-density modulator (PDM)), a FET driver stage 404,a high-power output switching stage 406, and an energy storage network408 which is shown being implemented using a fourth-order outputlow-pass filter (LPF) having a two-section ladder topology. The PWM 402generates PWM signals having pulse widths that are varied depending onthe magnitude of the input amplitude modulating signal AM(t), or, inembodiments of the invention in which a PDM is employed, the PDM signalshaving a pulse density that varies over time depending on the timevarying magnitude of the input amplitude modulating signal AM(t). Notethat in the description of the particular exemplary embodiment of theinvention below it is assumed that a PWM is used, but with theunderstanding that the invention is not limited to PWM and that in otherembodiments of the invention a PDM is used, instead.

The PWM signals from the PWM 402 are coupled to the FET driver stage404, which generates therefrom PWM drive signal PWM-HI and PWM-LO fordriving the high-power output switching stage 406. More specifically,the PWM drive signal PWM-HI switches a high-side FET 410 in thehigh-power output switching stage 406 ON and OFF and the PWM drivesignal PWM-LO drives a low-side FET 412 ON and OFF, according to themodulated pulse widths in the two drive signals. As will be understoodby those of ordinary skill in the art, since the PWM drive signalsPWM-HI and PWM-LO are complementary, i.e., 180 degrees out of phase, thehigh-side FET 410 is ON while the low-side FET 412 is OFF and viceversa. The switching voltage VDD_(SW) produced at the output switchingnode 414 of the high-power output switching stage 406 is filteredthrough the energy storage network 408 to produce a final DC-DCswitching converter output voltage VDD(DC-DC) that generally follows theinput amplitude modulating signal AM(t) and that has a cycle-by-cycleaverage proportional to the product of the duty cycle D(t) of the inputPWM and input DC voltage VDD, i.e., VDD(SW)∝D(t)×VDD, where T=1/f_(SW)is the period of the PWM switching frequency f_(SW) and D(t)=t_(ON)(t)/Tis the fraction of time in a given period the high-side FET 410 isswitched ON.

FIG. 5 is schematic drawing illustrating how the FET driver stage 404 ofthe DC-DC switching converter depicted in FIG. 4 is implemented in oneembodiment of the invention. The FET driver stage 500 includes anultrafast comparator 501, a high-gain differential pair 502, and firstand second drive interfaces 504 and 506. An input differential PWMsignal produced by the ultrafast comparator 501 is DC coupled to thehigh-gain differential pair 502, which amplifies the signal to increaseits peak-to-peak voltage swing. The first and second drive interfaces504 and 506 each includes an AC coupling capacitor, which removes the DCcomponent from the PWM signal it receives from the high-gaindifferential pair 502, and diodes that clamp the input-high andinput-low drive levels so that they always fall within the acceptableinput-high and input-low drive ranges of the drive interface FETs 510and 512. In one embodiment of the invention, the high-gain differentialpair 502, first driver interface 504, and second driver interface 506are integrated in a gallium nitride (GaN) integrated circuit (IC) chipand the drive interface FETs 510 and 512 and differential pair FETs 514and 516 comprise gallium nitride high electron mobility FETs (GaNHEMTs). GaN HEMTs are depletion mode FETs, i.e., ‘normally ON’ FETshaving a negative threshold voltage Vth, meaning that a gate-sourcevoltage V_(gs) less than (i.e., more negative than) the thresholdvoltage Vth must be applied across their gate-source terminals in orderto turn them OFF. The clamping diodes in the first and second driveinterfaces 504 and 506 ensure that happens. For example, for an inputPWM signal having a peak-to-peak voltage swing greater than or equal to4Vd (where Vd represents one forward diode voltage drop) and a sourcebias voltage VSS=−Vd, the diodes connected across the gate-sourceterminals of the drive interface FET 510 and 512 would each clamp theirgate-source voltage V_(gs) between an input-high drive levelV_(gs,H)=+Vd and an input-low drive level V_(gs,L)=−2Vd, i.e.,−2Vd<Vgs<+Vd, which is suitable for switching GaN HEMTs 510 and 512 witha threshold voltage Vth not less than (i.e., not more negative than)−2Vd fully ON and fully OFF, as illustrated in FIG. 6.

It should be mentioned that the FET driver stage 500 is designed so thatit does not require a transformer balun to distribute the inputdifferential PWM signal from the ultrafast comparator 501 on-chip to theGaN FET driver IC. The output peak-to-peak voltage swing of readilyavailable ultrafast silicon comparators is typically limited to lessthan 1Vpp but 3Vpp or more is needed to switch the GaN HEMTs 510 and 512in the driver interfaces 504 and 506 between fully ON and fully OFFstates. Although a transformer balun could be used to step up thelimited voltage swing provided by the ultrafast comparator 501 to thedesired 3Vpp swing, it is difficult to establish and maintain asymmetric drive signal when using a transformer balun. Symmetric drivesignals are desired since they preserve PWM and PDM signal quality. Thehigh-gain differential pair 502 and driver interfaces 504 and 506 areable to provide this symmetric drive capability without difficulty.Furthermore, because the high-gain differential pair 502 does notrequire precise DC biasing, the input PWM signal provided by theultrafast comparator 501 can also be directly connected to (i.e., DCcoupled to) the high-gain differential pair 502. The FET driver approachdepicted in FIG. 5 is therefore preferred since it obviates the need fora transformer balun. In addition to freeing up valuable printed circuitboard real estate that would otherwise be occupied by the transformerbalun, obviating the need for the transformer balun has the benefit ofincreasing the realizable controlled output voltage range from the DC-DCswitching converter 308, originally near to 20% to 80% of VDD and now upand beyond 10% to 90% of VDD. This is the result of the DC-DC switchingconverter 308 being able to realize a 10% to 90% or wider useful dutycycle range, whereas a driver approach that employs a transformer balunis only capable of realizing, at most, a 20% to 80% useful duty cyclerange.

In addition to helping recover the signal envelope from the switchvoltage produced at the output switching node 414, the energy storagenetwork 408 of the DC-DC switching converter 308 serves to filter outswitching noise and reduce ripple in the DC-DC switching converteroutput voltage VDD(DC-DC). As illustrated in FIG. 7, when the energystorage network 408 is implemented to include a fourth-order LPF theDC-DC switching converter 308 is able to realize a significantly wideroutput dynamic frequency response, compared to if a second-order filterwith the same switching noise suppression were to be used. To ensurethat this frequency response is not compromised and to avoid anypossibility of feedback stability problems, in one embodiment of theinvention the DC-DC switching converter 308 is configured to operateopen loop, i.e., without any feedback. It should be emphasized, however,that the DC-DC switching converter 308 can be operated closed loop solong as a compromise in bandwidth is acceptable. It should also beemphasized that whereas the LPF in the energy storage network 408preferably comprises a fourth-order LPF, any LPF of order greater thantwo (e.g., a third-order LPF) could be conceivably used to effectivelyincrease the dynamic frequency response of the DC-DC switching converter308.

The LAM 310 is responsible for removing any remaining ripple andresidual switching noise that may be present in the bucked DC-DC voltageVDD(DC-DC). In a preferred embodiment of the invention the LAM 310comprises a linear regulator having a silicon opamp 802 and GaN HEMT804, as shown in FIG. 8. The silicon opamp 802 serves as an erroramplifier that constantly adjusts its output voltage so that the voltageat its inverting input terminal equals the magnitude of the envelopesignal applied to its non-inverting input. Using its built-in powersupply rejection capability, the LAM 310 filters the DC-DC voltageVDD(DC-DC) supplied to it from the DC-DC switching converter 308, toproduce the final DPS voltage VDD(t) for the SMPA 302. The silicon opamp802/GaN HEMT 804 combination results in the LAM 310 being capable ofachieving an operating bandwidth of 900 MHz, i.e., nearly 1 GHz. Thisbandwidth is nearly 10× greater than if the LAM was made only fromsilicon semiconductor devices.

While various embodiments of the present invention have been presented,they have been presented by way of example and not limitation. It willbe apparent to persons skilled in the relevant art that various changesin form and detail may be made to the exemplary embodiments withoutdeparting from the true spirit and scope of the invention. For example,while the exemplary DPS 306 described above is well suited to serve as awideband envelope modulator in a polar modulator, it may also serve asthe DPS in envelope tracking (ET) amplifier that employs a linear PA andthat exploit the DPS to force the linear PA to always operate nearsaturation, where it is most energy efficient. Accordingly, the scope ofthe invention should not be limited by the specifics of the exemplaryembodiments of the invention but, instead, should be determined by theappended claims, including the full scope of equivalents to which suchclaims are entitled.

What is claimed is:
 1. A wideband envelope modulator, comprising: adirect current (DC)-to-DC switching converter including a pulse-widthmodulator configured to generate a PWM signal with modulated pulsewidths representing a time varying magnitude of an input envelope signalor a PDM signal having a time varying pulse density representing thetime varying magnitude of the input envelope signal, a driver stageconfigured to generate a PWM or differential PDM drive signal, ahigh-power output switching stage configured to be driven by the PWM orPDM drive signal, and a low-pass filter (LPF) having an order greaterthan two configured to filter a switching voltage produced at an outputswitching node of the high-power output switching stage; and a linearamplitude modulator (LAM) connected in series with the DC-DC switchingconverter.
 2. The wideband envelope modulator of claim 1, wherein theDC-DC switching converter is configured to operate open loop.
 3. Thewideband envelope modulator of claim 1, wherein the driver stagecomprises: a high-gain differential amplifier configured to amplify aninput differential PWM signal or input PDM signal; and a drive interfaceconfigured to receive an amplified PWM signal or amplified PDM signalfrom the high-gain differential amplifier, the driver interfaceincluding an alternating current (AC) coupling capacitor configured toremove a DC component from the amplified PWM signal, a depletion modefield-effect transistor (FET), and clamping diodes that clamp theAC-coupled and amplified PWM signal or amplified PDM signal between aninput-high drive level V_(gs,H) and an input-low drive level V_(gs,L)appropriate for switching the driver interface depletion mode FETbetween fully ON and fully OFF states.
 4. The wideband envelopemodulator of claim 3, wherein the input differential PWM or PDM signalis DC coupled to the differential input of the high-gain differentialamplifier.
 5. The wideband envelope modulator of claim 1, wherein theLAM comprises: an opamp having a first input configured to receive theinput envelope signal; and a power transistor having a gate or a basecoupled to the output of the opamp and a drain-source or acollector-emitter path configured between an output of the DC-DCswitching converter and a wideband envelope modulator output thatsupplies a final dynamic power supply voltage VDD(t).
 6. The widebandenvelope modulator of claim 5, wherein the power transistor comprises agallium nitride high electron mobility transistor (GaN HEMT) and theopamp comprises a silicon opamp.
 7. The wideband envelope modulator ofclaim 2, wherein the DC-DC switching converter has a 10% to 90% or wideruseful duty cycle range.
 8. A polar modulator, comprising: a phasemodulator configured to modulate a radio frequency (RF) carrier by aphase modulating signal PM(t) and produce a phase-modulated RF carrier;a wideband envelope modulator including an open-loop (DC)-to-DCswitching converter connected in series with a linear amplitudemodulator (LAM), the wideband envelope modulator configured to generatea wideband dynamic power supply (DPS) voltage VDD(t) from an input DCvoltage VDD and an input envelope signal; and a switch mode poweramplifier (SMPA) having an RF input port configured to receive thephase-modulated RF carrier, a power supply port configured to receivethe wideband DPS voltage VDD(t) from the wideband envelope modulator,and an RF output that produces a final amplitude- and phase-modulated RFcarrier suitable for transmitting over the air to a remote receiver. 9.The polar modulator of claim 8, wherein the DC-DC switching convertercomprises: a driver stage configured to generate a PWM drive signalhaving pulse widths that vary over time according to a time varyingmagnitude of the input envelope signal or a PDM drive signal having apulse density that varies over time according to the time varyingmagnitude of the input envelope signal; a high-power output switchingstage configured to be driven by the PWM drive signal or PDM drivesignal; and an output energy storage network including a low-pass filter(LPF) with order greater than two configured to filter a switchingvoltage produced at an output switching node of the high-power outputswitching stage.
 10. The polar modulator of claim 9, wherein the driverstage comprises: a high-gain differential amplifier configured toamplify an input differential PWM signal or input PDM signal; and adrive interface configured to receive an amplified PWM signal oramplified PDM signal from the high-gain differential amplifier, thedriver interface including an alternating current (AC) couplingcapacitor configured to remove a DC component from the amplified PWMsignal or amplified PDM signal, a depletion mode FET, and clampingdiodes that clamp the AC-coupled and amplified PWM or PDM signal betweenan input-high drive level V_(gs,H) and an input-low drive level V_(gs,L)appropriate for switching the driver interface depletion mode FETbetween fully ON and fully OFF states.
 11. The polar modulator of claim10, wherein the input differential PWM signal or input PDM signal is DCcoupled to the differential input of the high-gain differentialamplifier.
 12. The polar modulator of claim 8, wherein the LAMcomprises: an opamp having a first input terminal configured to receivethe input envelope signal; a power transistor having a gate or a basecoupled to an output of the opamp and a source-drain path orcollector-emitter path configured between an output of the DC-DCswitching converter and the power supply port of the SMPA.
 13. The polarmodulator of claim 12, wherein the power transistor comprises a galliumnitride high electron mobility transistor (GaN HEMT) and the opampcomprises a silicon opamp.
 14. The polar modulator of claim 8, whereinthe DC-DC switching converter has a 10% to 90% or wider useful dutycycle range.
 15. An apparatus, comprising: a phase modulator configuredto modulate a radio frequency (RF) carrier by a phase modulating signalPM(t) and produce a phase-modulated RF carrier; a dynamic power supply(DPS) configured to generate a wideband dynamic power supply (DPS)voltage VDD(t) from an input direct current (DC) voltage VDD and aninput envelope signal, said DPS including a DC-to-DC switching converterconnected in series with a linear amplitude modulator (LAM) and havingan output storage network that includes a low-pass filter (LPF) of ordergreater than two; and a power amplifier (PA) having an RF input portconfigured to receive the phase-modulated RF carrier, a power supplyport configured to receive the wideband DPS voltage VDD(t) from the DPS,and an RF output that provides a final amplitude- and phase-modulated RFcarrier suitable for transmitting over the air to a remote receiver. 16.The apparatus of claim 15, wherein the DC-DC switching converter furthercomprises: a driver stage configured to generate a PWM drive signalhaving pulse widths that vary over time according to a time varyingmagnitude of the input envelope signal or a PDM drive signal having apulse density that varies over time according to the time varyingmagnitude of the input envelope signal; a high-power output switchingstage configured to be driven by the PWM drive signal or PDM drivesignal; and an output energy storage network including a low-pass filter(LPF) with order greater than two configured to filter a switchingvoltage produced at an output switching node of the high-power outputswitching stage.
 17. The apparatus of claim 16, wherein the driver stagecomprises: a high-gain differential amplifier configured to amplify aninput differential PWM signal or input PDM signal; and a drive interfaceconfigured to receive an amplified PWM signal or amplified PDM signalfrom the high-gain differential amplifier, the driver interfaceincluding an alternating current (AC) coupling capacitor configured toremove a DC component from the amplified PWM signal or amplified PDMsignal, a depletion mode FET, and clamping diodes that clamp theAC-coupled and amplified PWM or PDM signal between an input-high drivelevel V_(gs,H) and an input-low drive level V_(gs,L) appropriate forswitching the driver interface depletion mode FET between fully ON andfully OFF states.
 18. The apparatus of claim 17, wherein the inputdifferential PWM signal or input PDM signal is DC coupled to thedifferential input of the high-gain differential amplifier.
 19. Theapparatus of claim 15, wherein the LAM comprises: an opamp having afirst input terminal configured to receive the input envelope signal;and a power transistor having a gate or a base coupled to an output ofthe opamp, and a drain-source or collector-emitter path configuredbetween an output of the DC-DC switching converter and the power supplyport of the PA.
 20. The apparatus of claim 19, wherein the powertransistor comprises a gallium nitride high electron mobility transistor(GaN HEMT) and the opamp comprises a silicon opamp.
 21. The apparatus ofclaim 15, wherein the DC-DC switching converter is configured to operateopen loop.
 22. The apparatus of claim 15, wherein the DC-DC switchingconverter has a 10% to 90% or wider useful duty cycle range.